1. Field of the Invention
The present invention relates to an optical memory element which optically performs at least one operation which is information recording, reading and erasing.
2. Description of the Prior Art
Recently, an optical memory element, which optically records, reads and erases information, has been drawing attention in various aspects as a high density, large capacity memory. Many studies have been made, particularly for an optical disc memory with disc shaped recording element which permits faster access than a memory with a tape element.
Since the information recording unit of the optical memory element is about 1 .mu.m.phi., it is essential to place a light beam at a predetermined position for the information reading or erasing operation. Various servo technologies have been employed for this purpose, because it is difficult to control the beam position depending only on mechanical accuracy.
For example, in an optical disc memory, a focus servo is used against the facial movement of the disc, and a tracking servo against the core movement. When the latter servo is used in a read-only type optical memory, beam position is controlled with reference to the previously recorded pits (normally, recesses formed in the PMMA or polycarbonate substrate). In the optical memory of an additive recording type (so called add-on-memory) or of the erasable memory type, however, guide track grooves and address signal pits indicating the positions of the grooves are normally formed in the substrate.
The construction of the conventional guide track grooves used in the optical memory of the add-on-memory type is described now with reference to FIG. 1.
FIG. 1 is a partially enlarged view of a conventional optical memory disc in section along the concentric or circulate guide track grooves formed in the disc.
In FIG. 1, 1 denotes a PMMA or glass substrate 1.about.1.5 mm thick, and 2 is a photosensitive resin layer (10 .mu.m.about.100 .mu.m thick 2P layer) with guide grooves 3 formed therein for storing information. 4 denotes address signal pits representing the positions of the guide track grooves 3. The depth of the pits 4 is normally .lambda./4n (.lambda. is the wave length of the reproducing laser beam, and n is the reflactive index of the 2P layer), so that address signals are optimally reproduced. The depth of the guide grooves 3 is .lambda./8n to permit storage of as many as possible tracking signals.
In addition to the above 2P method, it is also general to form the guide grooves 3 and the pits 4 integral with the PMMA or polycarbonate resin recording disc. In either method, however, moisture in the resin layer, on which a recording medium is disposed, can reach and deteriorate the recording medium.
To overcome the above problem, the inventor has presented a method for forming grooves directly in a glass substrate.
An example is described in copending U.S. patent application Ser. No. 606,534, "METHOD FOR MANUFACTURING AN OPTICAL MEMORY ELEMENT", filed on May 3, 1984 by Kenji OHTA, Junji HIROKANE, Hiroyuki KATAYAMA, Akira TAKAHASHI and Hideyoshi YAMAOKA, and assigned to the same assignee as the present application. The corresponding European Patent Application was filed on May 10, 1984 and assigned the Application No. 84303176.6. The corresponding Canadian Patent Application was filed on May 8, 1984 and assigned the Application No. 453,843.
According to the invention in the copending application, a resist film is disposed on a glass substrate, and a laser beam is applied onto the resist film to form a guide groove and address signal pit pattern in the resist film. The thus formed pattern is developed and, then, the developed resist film is directly disposed on the glass substrate by the reactive ion etching operation. With this method, however, it is difficult to form the guide grooves 3 and the address signal pits 4 of different depths, because their depth is determined by the amount of time to which the glass substrate is exposed to plasma in the reactive ion etching operation. Accordingly, the configurations of the guide grooves and the address signal pits must be determined, taking into consideration the address signal output, tracking output or signal output for reading these outputs.